The present invention relates to a display device for holographic reconstruction. In particular the present invention relates to a display device using in particular a planar combination of adjacent modulating pixels of a spatial light modulator device. Such display devices are required mostly for mobile applications like smart phones and tablet computers. However, other applications are also possible
Furthermore, the present invention relates also to a method of generating a holographic reconstruction and a spatial light modulator device applied in such a display device for generating at least one of a two-dimensional and/or three-dimensional representation of a scene or of content.
The present display device is adapted for displaying two-dimensional (2D) and/or three-dimensional (3D) images. It shall be understood that two-dimensional images or three-dimensional images also include two-dimensional or three-dimensional contents or movies. The field of application of the present invention includes preferably direct-view display devices for the three-dimensional presentation of holographic images.
In a commercially available flat TV display for the presentation of two-dimensional images or movies/videos, it is necessary to realize a bright and homogeneous illumination of the entire surface at high resolution. The spatial light modulator device which serves as display panel is required to emit the light in a large angular range. The information to be presented is written into the spatial light modulator device of the display device. The light which is emitted by an illumination unit comprising a light source unit is modulated with the information that is written into the spatial light modulator device, where the spatial light modulator device often at the same time serves as screen or display panel. It is therefore necessary to strictly ensure parallel incidence of the light beams onto the spatial light modulator device and to achieve a high refresh rate of the spatial light modulator device. To achieve a high quality of the three-dimensional presentation of the information written into the spatial light modulator device, a defined collimation of the wave fronts that are coupled out of the illumination unit is necessary in addition to a homogeneous illumination of the entire surface of the spatial light modulator device. This is of high importance for holographic presentations in the form of a reconstruction that is to be generated. The holographic information, which can for example be an object that is composed of object points of a three-dimensional scene, is encoded in the form of amplitude and phase values in the pixels of the spatial light modulator device. The encoded object points are generated by the wave field that is emitted by the spatial light modulator device.
A complex value which serves to modulate both the phase and the amplitude of a wave front cannot be displayed satisfactorily directly in a single pixel of a conventional spatial light modulator device. The modulation of only one value per pixel, i.e. a phase-only or an amplitude-only modulation, however only results in an insufficient holographic reconstruction of a preferably moving three-dimensional scene. A direct and thus optimal—in the sense of generalized parameters—representation of the complex values can only be achieved by a complex-valued modulation preferably at the same plane and at the same time in the spatial light modulator device.
Depending on the actual type of spatial light modulator device, various methods are known to achieve a simultaneous modulation of both parts of the complex values to be displayed.
For example, two separately controllable spatial light modulators can be combined and arranged very close to each other in order to simultaneously modulate both the amplitude and phase of coherent light. One spatial light modulator modulates the amplitude; the other one modulates the phase of the incident light. Further combinations of modulation characteristics are also possible with such an arrangement.
The light must first pass through one pixel of the first spatial light modulator and then through the corresponding pixel of the second spatial light modulator. This can be achieved for example in that the first spatial light modulator is imaged onto the second spatial light modulator by a large-area optical element, e.g. a lens, or the first spatial light modulator is imaged onto the second spatial light modulator by an array of small-sized lenses, or the two spatial light modulators are sandwiched together.
These combinations of two spatial light modulators which serve to achieve a complex-valued modulation have the disadvantage that the distance between the two spatial light modulators is much larger than their pixel pitch, i.e. the distance between two pixels.
A typical pixel pitch of a spatial light modulator for holographic applications is e.g. between 10 μm and 50 μm. In contrast, the distance between the two spatial light modulator panels in a sandwich arrangement is several 100 μm, in arrangements where one spatial light modulator panel is imaged onto the other, their distance is even larger.
Many types of spatial light modulators, such as liquid crystal (LC) spatial light modulators typically have an addressable layer of liquid crystals which is embedded between transparent glass substrates. Alternatively, in a reflective display device, the addressable layer is disposed between a transparent glass substrate and a reflective glass substrate. The glass substrates typically have a thickness of e.g. between 500 μm and 700 μm.
A sandwich structure for a complex-valued modulation can be created in that a single phase-modulating spatial light modulator and a single amplitude-modulating spatial light modulator are arranged with their glass substrates one after another. When a pencil of rays coming from the addressable layer of a pixel of the phase-modulating spatial light modulator falls on the addressable layer of a pixel of the amplitude-modulating spatial light modulator after the passage through the glass substrates, it would already be broadened at the aperture of this pixel by diffraction effects so that cross-talking of pencils of rays of adjacent pixels would occur.
When using imaging elements, there is the challenge that exactly one pixel of the first spatial light modulator must be imaged onto one pixel of the second spatial light modulator across the entire surface of the spatial light modulators. This requires optical systems which exhibit extremely little distortion. Such requirements can hardly be fulfilled in practice. This is why cross-talking between adjacent pixels also takes place when an imaging technique is employed.
Cross-talking can be even worse if the two spatial light modulator panels, the optical imaging system or the light sources are not perfectly aligned in relation to each other.
Furthermore, if spatial light modulator panels are combined and disposed very close to each other, such arrangements are susceptible to errors when diffraction of the incident wave field will take place at the first plane of diffracting apertures, which is e.g. a phase spatial light modulator forming the first plane of the sandwich-type complex spatial light modulator. The diffracted light from a pixel placed in the first spatial light modulator panel, can propagate to adjacent, non-corresponding pixels of the second spatial light modulator panel. In other words, e.g. 80% of the light propagating behind a phase modulating pixel of the first panel can illuminate the related amplitude modulating pixel of the second panel. Other 10% cannot hit a transparent part of the amplitude panel which means that this 10% can be blocked by the absorbing parts of the apertures. And the remaining 10% can illuminate adjacent amplitude pixels which means to generate crosstalk within the sandwich. This crosstalking deteriorates e.g. the reconstruction quality of a holographic display device because this corresponds with a wrong combination of amplitude and phase values when complex values are represented by the spatial light modulators.
As a consequence, the distance between phase and amplitude modulating planes has to be preferable less than ten times the smallest pixel pitch, which is present along a coherent direction. For instance, in the case of using one-dimensional encoded sub-holograms, which can be the case for holographic TV, the light illuminating the spatial light modulator is coherent along one direction only, which can be e.g. the vertical direction.
In display devices used for holographic reconstruction a complex modulation of sufficiently coherent light emitted by the illumination unit has to be achieved to control the amplitude and the phase of pixels of a spatial light modulator device, herein after referred to as SLM, independently from each other. The use of a respective spatial light modulator device is therefore necessary.
A possibility to obtain the complex modulation by an SLM is to laterally combine adjacent phase modulating pixels and amplitude modulating pixels.
Such a lateral arrangement of adjacent amplitude modulating pixels and phase modulating pixels is disclosed in WO 2009/080576 A1. This document describes a controllable light modulator comprising a number of macro-pixels of at least two pixels which are arranged next to each other, where retro-reflective elements are provided. A retro-reflective element has two reflective surfaces, which are in parallel in the vertical direction. The reflective surfaces are arranged without gap under a given angle of 90° deg such that they form a prism with a substrate layer in which the retro-reflective element is arranged and such that they reflect an incident part of a wave field.
In general, a pixel combining arrangement in an SLM can make use of tilted surfaces, which can be realized e.g. as one-dimensional (1D) or two-dimensional (2D) prism structures, which can be realized by using e.g. molding or imprint technologies.
The precision of the prism angles, which can be realized, is significantly less than 1/10° deg. And even an angular tolerance of 0.5° deg requires significant technical effort if realized at a display size area and with a prism spacing of <100 μm.
It is not sufficient to focus onto the combination of a phase modulating pixel and an amplitude modulating pixel, which are placed sequentially along the optical path of the illuminating light, regardless if a standard-type display sandwich is used or adjacent phase modulating pixels and amplitude modulating pixels are combined in order to form a functionally sequential phase and amplitude-SLM sandwich. This is only one boundary condition. To understand the requirements the entire function has to be understood, which is the generation of real or imaginary object points within the space of a frustum, which is spanned by a viewing window (VW) where a user's eye is placed and the SLM and even might continue behind the SLM, and which has a pyramid-like structure. The generation of the object points requires a collimated illumination. To show high definition content means to limit the angular spectrum of plane waves (ASPW) of the illumination to 1/60° deg.
Furthermore, there is a difference between the angular spectrum of plane waves and the angular distribution, which is added by the micro prism array e.g. used in WO 2009/080576 A1. The angular spectrum of plane waves has to be ≤ 1/60° deg but the angular variation, which is added by the prisms, has not to be as small as this value of the angular spectrum of plane waves and can be e.g. >0.5° deg. An individual calibration of local pixels of the SLM is not required if it is as small as 1/60° deg. But if the angular variation, which is introduced by the prisms, significantly exceeds 1/60° deg, then a local calibration is required in order to provide the exact phase values within the entrance plane of the human eye, which means the design values of the complex-valued wave field. In other words, a calibration of all pixels or at least of a very high number of sampling points has to be carried out. The plane of the measurement can be the display plane or close to the display device. The locally measured wave field, which means locally measured phase and amplitude distributions, can be propagated to a viewing window by using e.g. a Fresnel transformation. In other words, this can be done by using calculations of phase and amplitude distributions. However, the complex-valued wave field which is emitted by individual pixels or rather complex-valued pixels of the SLM can also be measured within the viewing window itself without using calculated propagation. The viewing window is a visibility region in a user plane. If the position of the viewing window in the user plane coincides with the eye of a user and then the user looks through the viewing window, he can observe the reconstructed scene.